The present invention relates to a flat synthetic resin having a flattened portion or region partially or as a whole through the fiber; and a method for the production of the synthetic fiber. More specifically, the present invention relates to a flat synthetic resin suitably for use in the production of a non-woven fabric having a high strength and good formation even at a low basis weight as well as a simple and efficient method for preparing the flat synthetic resin.
In general, a non-woven fabric is produced by binding a web-like or sheet-like fibrous aggregate, as a base material, with a resin binder (hereunder also simply referred to as xe2x80x9cbinderxe2x80x9d) or adhesive fibers or by treating the base material according to the needle punch or water-jet technique to thus make fibers intersect with each other. However, if the product should have a desired strength, the foregoing method, which makes use of a binder or adhesive fibers, in particular, the method of making use of a binder is adopted. In this case, the binder is adhered to the intersection points and fibers are bonded together to thus improve the strength of the resulting non-woven fabric.
Poly(p-phenylene-benzobisoxazole) (hereinafter referred to as xe2x80x9cPBOxe2x80x9d), oly(p-phenylenebenzobisoxazole), poly(p-phenylenebenzobisthiazole), poly(p-pheynylenebenzobisimidazole), poly(2,5-benzoxazole) and poly(2,6-benzothiazole) are examples of polybenzazoles (hereinafter referred to as xe2x80x9cPBZxe2x80x9d) or hetero ring-containing polymers. For example, PBO can be obtained by the polymerization reaction of diaminoresorcinol with terephthalic acid in polyphosphoric acid as a solvent. The PBO fiber is prepared by spinning this polymerized dope according to a dry or wet spinning technique, washing the spun product with water and then drying the fiber thus prepared. The fiber shows quite high molecular orientation even at a slight elongation, it can therefore easily take its full length-stretched chain structure, and shows strength and elastic modulus higher than those observed for the carbon fiber. Moreover, it also shows the highest levels of the thermal decomposition temperature and flame retardancy among the existing organic fibers and it has been expected as a super fiber for the next generation and there have been desired for the development of a wide variety of applications of the fiber.
An example of commercially available PBO fibers is ZYLON (available from Toyobo Co., Ltd.) and examples of shapes of the fiber are spun yarns, filaments, staples, chopped fibers and pulp. The term xe2x80x9cchopped fiberxe2x80x9d used herein means a product obtained by binding continuous fibers in a bundle and then cutting the bundle of the fibers into pieces having a predetermined size and the term xe2x80x9cpulpxe2x80x9d herein used means a product obtained by beating chopped fibers to thus fibrillate the fibers partially (such as the surface) or completely. It has been intended to use them as friction materials, reinforcing fibers for gaskets and FRP reinforcing fibers. However, it is also possible to form a non-woven fabric using chopped fibers. For instance, Japanese Patent Kokai No. 2000-165000 discloses that the non-woven fabric prepared from PBO fibers will be full of promise as a base material for printed circuit boards.
However, the non-woven fabric of PBZ fibers such as PBO fibers is poor in the working characteristics in, for instance, slitting. Moreover, a laminated board comprising, as a base material, a non-woven fabric of PBZ fibers is poor in the working characteristics, for instance, the suitability for the perforation of the laminated board with laser beams and the suitability for drilling the same. This becomes a cause of a serious problem encountered in, for instance, the processing of a laminated board for printed circuit boards.
As a means for solving the foregoing problems, it would be conceivable to reduce the thickness of such a fabric, while maintaining the basis weight thereof at a low level. In particular, there has recently been desired for reducing the thickness of a printed circuit board in line with the recent trend that electronic machinery and tools are made lighter and smaller. Therefore, there has been an intensive demand for the reduction of the thickness of the non-woven fabric as a base material. However, the reduction in the thickness of a non-woven fabric, while reducing the basis weight thereof, may become a cause of various problems such that the tensile strength of the fabric is reduced, that the fabric has an increased irregularity in the formation thereof, and that the number of pinholes formed in the resulting non-woven fabric increases.
As a means for solving this problem, it may be conceivable to reduce the diameter of the fibers. Such a method permits not only the improvement of the working characteristics of the fibers, but also the augmentation of the number of fibers per unit basis weight, and this would lead to the achievement of effects of reducing the irregularity in the formation and of reducing the number of pinholes formed. Moreover, it would be expected that the strength of the resulting non-woven fabric is improved because of an increase in the relative bonding area. However, the smallest diameter of the chopped fibers presently commercially available is 1.5 d (about 11.6 xcexcm), but fibers having such a diameter never provides non-woven fabric sufficient in the working characteristics.
Furthermore, it may, Likewise be conceivable to incorporate pulp into such a non-woven fabric, but a large amount of pulp need be incorporated into the non-woven fabric to improve the working characteristics thereof. In this respect, if the amount of the pulp to be incorporated therein is up to about 20%, it shows the effect of improving the strength of the resulting non-woven fabric, but if it is used in an excess amount, the pulp rather becomes a cause of the reduction in the strength and it is apt to generate flocks and this in turn impairs the texture of the resulting fabric.
Recently, there have been proposed a variety of high strength fibers, which have never been experienced conventionally, such as aromatic polyamide fibers (aramid fibers), and ultra high strength polyethylene fibers, polyarylate fibers, as well as polybenzazole (PBZ) fibers such as PBO fibers, and non-woven fabrics produced from such high strength fibers have correspondingly been put on the market. In particular, in the non-woven fabric produced from such fibers having heat-softening properties such as meta-aramid fibers, polyarylate fibers and ultra high molecular weight polyethylene fibers, fibers can be fusion-bonded together by subjecting them to a calendering treatment at a temperature of not less than the softening temperature of the fibers to thus form a non-woven fabric having a very high strength.
In a non-woven fabric prepared using fibers, which are substantially free of heat-softening properties and any self-adhering ability such as para-aramid fibers and polybenzazole (PBZ) such as PBO fibers, however, the strength of the portions bonded using a binder governs the resistance to breakage of the resulting non-woven fabric. More specifically, the strength of the non-woven fabric comprising these fibers as main components is dominated by the bonding strength established by the binder used.
It would be conceivable to simply increase the amount of the binder used as a means for improving the strength of such a non-woven fabric. However, each binder should in general be used in an amount falling within an appropriate range. More specifically, the strength of the resulting non-woven fabric is reduced if the amount is beyond the foregoing appropriate range. Moreover, if the amount of the binder is too large, the resulting non-woven fabric loses the flexibility thereof or it may often lose the desired its quality depending on the applications thereof.
On the other hand, it would also be conceivable to increase the contact area between fibers by flattening the fibers. For instance, Japanese Kokoku No. Hei 6-60035 introduces a glass fiber having a flat cross section. More specifically, it would be expected that the strength of the non-woven fabric can be improved by increasing the contact area between fibers, in case of fibers such as glass fibers, which may be chemically bonded with a binder through a coupling agent, in case of the foregoing fibers having thermally fusion-bonding ability or in case of those having self-adhering properties such as cellulose fibers.
In case of fibers, which do not have an ability of chemically bonding with a binder, a fusion-bonding ability through heat softening and an ability of self-adhesion such as para-aramid fibers and PBZ fibers, however, there is no means for ensuring the bonding strength other than the coverage of the contact points between the fibers with a binder.